1. Field of the Invention
The present invention relates to injection molding machines, and more particularly to an injection molding apparatus capable of producing multiple layer plastic articles having a sandwich layer configuration.
2. Description of the Related Art
The sandwich method of injection molding enables the production of plastic articles whose cross-section has a three-layer configuration. The outer or "skin" surface is one material, and the inner or "core" structure consists of another material. This construction has often been used for thick walled components whose cores consist of foamed material, and in applications where an inexpensive material can be used for the core of an article (replacing an expensive material) the requires special properties at the surface. More recently, there has been an increased emphasis on recycling which has prompted many molders to look for ways to use reground material. The sandwich method of molding can effectively use reground material as the core material since it still has adequate physical properties. This construction takes advantage of the fact that the outer (skin) material is relatively thin, thereby helping reduce the cost of the molded article by minimizing the amount of virgin material.
From a material processing point of view, the sandwich configuration is achieved when the two materials are conveyed one after the other into the mold cavity. When the materials are injected under laminar flow conditions, the injected melt begins to solidify immediately when it has made contact with the mold wall so that the melt at the center of the flow advances faster than the melt in the vicinity of the mold wall. Accordingly, material injected later displaces material injected earlier, particularly in the middle of the cross-section of the flow stream, while the melt that contacts the cold mold walls solidifies to form and maintain the surface layer of the part. When the subsequent melt flow includes a second material, the sandwich or three-layer configuration is created.
Because of the laminar flow pattern, it is not possible to injection mold a sandwich configuration by introducing both melts into the mold cavity at the same time. Simultaneous injection would result in unacceptable surface since the core material would contact the walls of the mold cavity and mix with the skin material at the surface of the molded article. To ensure the proper surface layer the skin material is always injected first with the sandwich molding method; the core material then follows at a short timed overlap. The overlap in injection of the two materials is necessary to prevent the melt flow front from momentarily stopping which would result in surface blemishes ("knit lines". With this type of timed sequence, it is immaterial whether the melts are brought together in a concentric or parallel nozzle. In all cases, however, the second (core) material must be a subsequent melt stream that flows through the flow cross-section of the first injected material in order to form the sandwich configuration.
Typically, injection molding apparatus designed to produce the sandwich configuration have employed two injection units, controlled to inject the respective component materials into the mold cavity in the desired timed sequence. Machines of this type have disadvantages involving the high initial cost of the required components and associated control capability for close operation monitoring to ensure that the volume flow of material is constant through the nozzle into the cavity when changing from the skin material to the core material. More recently, it has been discovered that the sandwich molding method does not necessarily require that the two melts be injected at different times from separate injection cylinders. It is also possible to layer the melts in a single cylinder and then inject these "stacked" melts into the mold cavity in a single injection stroke. This method of sandwich molding has the advantage of allowing the injection process to be set in the same way as standard (single material) injection molding.
The apparatus to perform the single injection method would typically comprise a standard injection molding machine having a main injection unit that receives plastic melt from one or more auxiliary plasticizing units. The transfer of the skin and core material into the main injection unit is controlled to achieve a total shot of the two stacked materials in the desired proportions. For this method, the auxiliary plasticizing unit can be a non-reciprocating screw extruder connected to a main injection unit having a reciprocating screw for injection. Alternatively, the auxiliary unit could be a second reciprocating screw injection unit where the screw operates in the normal manner to transfer material into the main injection unit; or two extruders supplying an injection accumulator.